In recent years, reduction of the automobile body weight is demanded as one of the measures to reduce fuel consumption. For the purpose of reducing the car body weight, it is requested to reduce the thickness of the cold-rolled steel sheet, which accounts for about 40% of the car body weight among the component members of the car body, by increasing tensile strength thereof and imparting a higher dent resistance thereto.
Conventionally, such a high-strength cold-rolled steel sheet has been manufactured either by a method comprising subjecting a cold-rolled steel strip added with solid-solution element to a batch annealing and strengthening said strip by the effect of this solid-solution element, or by a method comprising subjecting a cold-rolled steel strip added with elements forming carbides and nitrides to a batch annealing and strengthening said strip by the effect of precipitates of said elements forming carbides and nitrides. A steel sheet manufactured by any of such methods has however been problematic in the low productivity and the high manufacturing costs.
As a measure to solve the above-mentioned problems, there is known the following method for manufacturing a high-strength cold-rolled steel sheet through continuous annealing:
(1) A method for manufacturing a high-strength cold-rolled steel sheet excellent in accelerated ageing property, disclosed in Japanese Patent Publication No. 41,983/79 dated Dec. 11, 1979, which comprises:
According to the above-mentioned method, it is possible to manufacture a high-strength steel at a high productivity with low costs. However, the steel sheet manufactured by this method, having a high tensile strength of from 40 to 80 kg/mm.sup.2, has problems because the achievement of such a high tensile strength resulted in a poorer press-formability.
Because of this inconvenience, the application of a high-strength cold-rolled steel sheet to the automobile body is limited to those members particularly requiring a high strength such as a bumper and a guard bar, and the interior members of the car body in which the strain produced during the forming process does not form a difficulty. For the automobile outer shell, which is the most important application of the cold-rolled steel sheet in consumption, an ordinary deep-drawing quality mild cold-rolled steel sheet is used at present, since it is impossible to manufacture a high-strength cold-rolled steel sheet excellent in both press-formability and dent resistance, in spite of the remarkable advantage of using a higher tensile strength steel sheet.
The above-mentioned high-strength cold-rolled steel sheet for automobile outer shell should preferably have a tensile strength of from 35 to 50 kg/mm.sup.2. The batch-annealing type phosphorus-containing aluminum-killed cold-rolled steel sheet having a prescribed phosphorus content is known as a cold-rolled steel sheet having the above-mentioned tensile strength, and at the same time, having unimpaired formability. This batch-annealing type P-containing Al-killed cold-rolled steel sheet is manufactured by the utilization of the contribution of the phosphorus content to the achievement of a higher tensile strength without impairing deep drawability. For example, in order to obtain a high-strength cold-rolled steel sheet having a tensile strength of 40 kg/mm.sup.2, the phosphorus content should be at least from 0.07 to 0.10 wt.%, and the dissolution of phosphorus in solid-solution form into ferrite brings about a yield strength of from 28 to 30 kg/mm.sup.2.
In the above-mentioned batch-annealing type P-containing Al-killed cold-rolled steel sheet, almost no bake-hardening is produced during the baking process of paint. Dent resistance of the batch-annealing type P-containing Al-killed cold-rolled steel sheet is therefore based solely on the above-mentioned yield strength thereof. In addition, in the batch-annealing type P-containing Al-killed cold-rolled steel sheet, when press-formed, the yield strength increased by the addition of phosphorus leads to an increase in the amount of spring-back as well as to the deterioration of shape-freezability. Furthermore, this steel sheet, being manufactured through batch annealing, has problems because of low productivity and increased manufacturing costs.